Generator rotors have a cylindrical rotor body from which two shafts extend at each rotor end. The rotor is angularly divided into teeth regions and pole regions. The teeth regions have a series of teeth that are defined by longitudinal slots cut into the rotor. The slots contain field windings and wedges for hold the windings in place against centrifugal force caused by the rotor rotation.
Referring to FIG. 3, windings 120 axially extend beyond the rotor teeth 116 at each end of the rotor body where they bend to form winding end turns. Because the end turns are in an area that lacks teeth 116 and wedges 119 to restrain centrifugal force, an annular retaining 114 ring is placed around the end turns and rotor end so as to restrain the end turns against centrifugal force. The retaining ring 114 is longitudinally held in place by a ring key 110 having a radial split 28, as shown in FIG. 2, which fits into opposing, circumferential grooves 130a, 130b in the retaining ring 110 and the rotor 112, respectively. In FIG. 3, components unrelated to the present invention, such as filler blocks and damper bars, are omitted for clarity.
During operation of the generator, the rotor 112, especially the windings and end turns, heat up. Thermal expansion causes the end windings to push axially outward against the retaining ring 114, resulting in an axially outward force, termed "friction force." Because the ring key 110 limits the retaining ring 114 from longitudinally moving relative to the rotor 112, the ring key 110 transmits to the rotor 112 the friction force, as well as other longitudinal forces having the same direction. Because the conventional ring key 110, shown in FIG. 2, has approximately uniform thickness and hardness throughout it circumference, the friction force is approximately evenly transmitted to the teeth regions and pole regions of the rotor. The portion of the friction force that is transferred to the rotor teeth is problematic because the teeth are already subject to high radial stress caused by centrifugal force from the windings and wedges. Such high stress has caused cracking in the tooth tops.
There is, therefore, a need for a generator rotor having a ring key that diminishes the longitudinal friction force in the teeth region of the rotor.